The present invention relates to a method of inhibiting corrosion of ferrous metals in an aqueous system. More particularly, the invention is directed to the use of one or more poly(amino acids) to inhibit corrosion.
Ferrous metals such as carbon steel and stainless steel are widely used in the construction of equipment associated with aqueous systems. By "aqueous systems" we mean equipment or process operations, containing ferrous metals, which are contacted with aqueous fluids on a regular basis. Typical aqueous systems include for example water treatment systems such as cooling water systems, boilers, heat exchange equipment, reverse osmosis equipment, oil production operations, flash evaporators, desalination plants, paper making equipment, steam power plants, and geothermal systems. Aqueous systems also include for example equipment used in detergent applications such as laundry machines and automatic dishwashing machines. A common problem in aqueous systems is that the aqueous fluids corrode the ferrous metals in the aqueous systems.
The corrosion of ferrous metals is believed to be caused by agents, such as oxygen, present in the aqueous fluid, which oxidize the ferrous metals.
The corrosion in aqueous systems can be accelerated by many factors. For example, contaminants such as cations, like magnesium and calcium, and anions, like sulfate and chloride, can increase the rate of corrosion. The rate of corrosion is also increased when aqueous systems are contacted with water from natural resources such as from sea, lakes, and rivers. It is believed that certain microorganisms in water from natural resources interact with the ferrous metals to accelerate corrosion.
Corrosion can also become more severe when the pH of the aqueous fluid is lowered, or when the temperature of the aqueous fluid is raised. Additionally, evaporation of water in aqueous systems, concentrates contaminants in the aqueous fluid, and can accelerate corrosion.
There may be several different types of corrosion encountered in aqueous systems. For example, aqueous systems may have uniform corrosion over the entire metal surface. The aqueous system may also have localized corrosion, such as pitting or crevice corrosion, where the corrosion is found only in certain locations on the ferrous metals.
Corrosion in aqueous systems can be a serious problem. The corrosion causes the ferrous metal to weaken and deteriorate. Eventually, if the corrosion is severe, the ferrous metal parts or possibly the entire aqueous system must be replaced. Replacement of parts or the aqueous system can be expensive and can also lead to a significant loss in operating time.
Corrosion inhibitors are commonly added to aqueous systems to mitigate corrosion of the ferrous metals. Common corrosion inhibitors include for example water soluble zinc salts, phosphates, polyphosphates, phosphonates, nitrates, molybdates, tungstates, silicates, ethanolamines, and fatty amines. Recently, it has been desired to use partially or completely biodegradable corrosion inhibitors in aqueous systems. For example, U.S. Pat. No. 4,971,724, to Kalota, hereinafter referred to as "Kalota" discloses the use of certain amino acids and certain homopolymers of amino acids such as aspartic acid and poly(aspartic acid) as corrosion inhibitors.
However, the amino acids and homopolymers of amino acids disclosed in Kalota are only effective as corrosion inhibitors in systems having a pH greater than 9.5 (25.degree. C.). Below a pH of 9.5, the amino acids and homopolymers of amino acids in Kalota are disclosed to promote corrosion in comparison to systems having no corrosion inhibitor. Since many systems, such as cooling water systems have a pH lower than 9.5, it is desirable to provide corrosion inhibitors that are effective at a pH less than 9.5.
Additionally, some systems, such as oil production operations, use aqueous fluids where the pH varies depending on the location of the operation. For example, in oil production operations, downhole, or below ground, the aqueous fluid has typically a pH less than about 6. However topside, or above ground, the aqueous fluid often has a pH greater than 6. Therefore, it is desirable to develop corrosion inhibitors which work effectively over a wide range of pHs from 3 to 12.
In addition to the need to develop corrosion inhibitors which are effective over a wide range of pHs, there is also a desire to replace water soluble silicates which are currently being used in detergent applications as corrosion inhibitors. The water soluble silicates are problematic because they can cause processing problems when preparing detergent formulations used in detergent applications.
The problem addressed by the present invention to provide corrosion inhibitors which can replace water soluble silicate corrosion inhibitors in detergent applications. The present invention also addresses the problem of providing corrosion inhibitors which are effective over a wide range of pHs from 3 to 12.
Accordingly, the present invention provides corrosion inhibitors comprising poly(amino acids) which are useful in detergent applications to replace water soluble silicate corrosion inhibitors. The present invention also provides corrosion inhibitors comprising one or more homopolymers of amino acids and one or more pyrophosphates which are useful in aqueous systems operated over a wide range of pHs.